Microcellular polyurethane elastomers have been used as a vibration isolation material, a shock absorber, etc. They have conventionally been obtained by a process comprising reacting a polyester polyol having a number average molecular weight of 1,000 to 3,000 which is obtained by polycondensation between at least one alkylene oxide (e.g., ethylene oxide, propylene oxide or 1,4-butylene oxide) adduct of a polyol (e.g., ethylene glycol, propylene glycol, 1,4-butylene glycol, glycerin or trimethylolpropane) and at least one organic acid (e.g., malonic acid, maleic acid, adipic acid or terephthalic acid) with a polyisocyanate, e.g., 2,4-tolylene diisocyanate (2,4-TDI), 2,6-tolylene diisocyanate (2,6-TDI), a mixture of 2,4-TDI and 2,6-TDI, diphenylmethane-4,4'-diisocyanate (MDI), naphthylene-1,5-diisocyanate (NDI) or 3,3'-dimethyl-4,4'-biphenylene diisocyanate (TODI) to obtain an isocyanate (NCO)-terminated prepolymer and reacting the NCO-terminated prepolymer with a foaming component comprising water, a catalyst, a foam stabilizer, etc. while stirring (cf., unexamined published Japanese patent application No. 57-100121).
Of the elastomers obtained by the above process, those prepared by using NDI as a polyisocyanate component are excellent in mechanical strength and durability under repeated high loads, such as resistance to fatigue failure or permanent set, and are therefore useful as a cushion material for an auxiliary spring of automobiles. However, NDI is expensive because it is produced chiefly for use as an intermediate material for synthesizing pharmaceuticals, dyes, etc. on a relatively small scale with a narrow market. Further, the NCO-terminated prepolymer obtained by the reaction between a polyester polyol and NDI has a short pot life and poor workability in mixing with a foaming component for molding and curing due to its high viscosity. On the other hand, MDI, which is a general-purpose polyisocyanate compound, is inexpensive, and the NCO-terminated prepolymer obtained by using MDI has a low viscosity, exhibiting satisfactory workability in foaming and curing as well as a long pot life. However, a polyurethane elastomer prepared from this NCO-terminated prepolymer has insufficient mechanical strength and insufficient durability.